Arcing in electrical systems is a well-known but unwanted phenomenon, typically resulting from either poor installation procedures for system components, or breakdown of conductors or insulators in the system, creating an arc pathway between two conductors or to ground. Arcs may damage electrical system components immediately or over time and may cause potentially detrimental circuit breaks. If an arc has sufficient current and voltage, it may become a sustained arc that is either substantially constant or recurs at regular or irregular intervals. A sustained arc is desirable for applications such as welding, but in other applications an unwanted sustained arc can melt, corrode, or otherwise damage system components, and can reduce the overall performance of the system. In some electrical systems, a sustained arc may be produced at very small current levels.
Electrical systems often employ circuit protection devices, including arc detectors that place the system in a “fault” condition, such as by tripping a circuit breaker or initiating an alarm when the detector detects an arc that exceeds a certain threshold or contacts a certain component. Conventional circuit breakers are designed to protect electrical circuits by detecting overloads and short circuits. The amount of current transferred in these situations is high, so these devices have a low sensitivity to current variations and thereby avoid false alarms that would break the circuit without need. In contrast, a residual-current device (“RCD”) is configured to disconnect an electrical circuit when the device detects an excessive imbalance in system current that can be caused by arcing transfer of current to a conductor that normally does not carry system current. RCDs, including ground fault interrupters (“GFIs”), earth leakage circuit breakers “ELCBs”, safety switches, and trip switches, are configured with a much lower sensitivity than a conventional circuit breaker and are able to detect arc-induced erratic circuit behavior that does not trip a breaker. At such a low sensitivity, the RCD must be further configured to differentiate between an arc-induced current variation and one caused by normal circuit operation, such as the actuation of a switch, the activation or deactivation of a motor, or the sudden removal of a load by unplugging or other means, in the electrical system. This adds cost and complexity to RCDs, and false positive circuit interruption remains a major drawback for existing RCDs. Yet a third type of device, an arc fault circuit interrupter (“AFCI”), may detect variations in the current in both frequency and time which are not characteristic of any regular electrical loads.
However, while they can be more sensitive than some RCDs and can also detect arcs between load and neutral terminals not involving ground, experimental tests have shown that AFCIs are still not sufficiently sensitive to detect a sustained arc in some electrical systems. For example, in long parallel electrical heating cables, the part of the system affected by the arc is small compared to the overall heater, leading to a small electrical signature from the sustained arc. In particular, attempts to deploy known commercial AFCIs to detect sustained bus-to-bus arc faults in self-regulating polymer-based heating cables have failed. A further advantage of using sub-harmonic frequencies over frequencies at or greater than the line frequency is the weaker attenuation over long and lossy transmission lines for lower frequencies.
Another disadvantage of existing AFCIs can be that they rely on detecting frequency anomalies at frequencies which can be in the kHz or MHz range, but at any rate which are greater than the line frequency. In case of attenuating transmission lines which either lead to or are part of the electrical system, existing AFCIs may not detect arcing signatures that are better observable at frequencies below the line frequency.